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Old 05-01-2010, 12:49   #1
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Yamaha Acceleration Problem

So I have fixed one problem on the outboard, now for the next.

The motor starts very well, will go into forward and drive the dinghy. If I go to full throttle, it will accelerate out of control. If I stay in half speed all is good.

Could this be a shear pin that is broken? Also I went to the dealer and looked at a pin and I am use to a shear pin that is not entirely straight, an indentation where it is designed to break. (Snow blowers)

Confession:
I have recently had the forward, reverse, neutral control arm replaced, it broke off after the dinghy got washed under a pier. I also have touched bottom a couple of times trying to cut a point a little too close.

I have a Yamaha 5hp, 2001, model 5MSHZ.

Any suggestions?
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Old 05-01-2010, 12:57   #2
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Are you saying it feels like the engine is accelerating without a subsequent increase in propeller RPM?

If so, the rubber donut inside the propeller hub may be slipping. It is designed to slip if your propeller ever hits something. It is designed to protect the engine and drive train. The only solution I know of is to replace the propeller. The propeller goes over a splined shaft and is held in place by a nut that is prevented from turning by a bendable nut cap or a shear pin. The nut either works or it does not.
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Old 05-01-2010, 13:00   #3
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Yes, after about accelerating half way, it will rev VEY high if I increase the gas and no increase in speed.
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Old 05-01-2010, 13:03   #4
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On this engine, does the cotter pin you see holding the nut to the prop work like the shear pin?
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Old 05-01-2010, 13:06   #5
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That does sound like a slipping hub, landonshaw. I don't know of any recent engines that use a shear pin on the shaft (a cotter pin, of course, is still standard)- the rubber hub is the standard now, and it behaves very much as you describe when it starts to fail. Ventilation (sucking in air from the surface) or cavitation (vacuum bubbles on the trailing face of the blades- rare on a motor so small) can also cause similar symptoms, but if either of these were at fault, you'd have noticed when the engine was first installed.
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Old 05-01-2010, 13:07   #6
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I would bet you have a rubber mounted prop, and its slipping with the increase in power-what eles could it be??
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Old 05-01-2010, 13:08   #7
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I think your prop is slipping around the rubber donut. The way to test this is to lock your crank shaft in place with your engine in gear. Then put a good sized pipe wrench on the prop itself and see how easily it turns. If it turns real easy then you have found the problem. Don't go overboard with the torque..you don't want to break anything.
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Old 05-01-2010, 14:07   #8
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Are you planing

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Yes, after about accelerating half way, it will rev VEY high if I increase the gas and no increase in speed.
Are you planing? Or just pushing water at full throttle.
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Old 05-01-2010, 18:25   #9
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I am pushing water.
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Old 05-01-2010, 18:28   #10
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Are you able to replace the rubber donut or is it a new prop?
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Old 06-01-2010, 11:13   #11
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Let me understand

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I am pushing water.
Have you been able to plane?
Its 5hp, How big is the boat? I have a 4 hp 2 stroke Merc. I can only plane with one in the boat. If I push it any further when loaded I only push water but don't plane.

If your just reaching hull speed you wont be able to plane and wont go any faster with full throttle?

If you were planing before with the same load then you may have a problem.

S
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Old 06-01-2010, 12:40   #12
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Unfortunately, you must replace the entire prop but they are not expensive. The dealer should be able to sell you the prop that is consistent with your application.
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Old 08-01-2010, 16:01   #13
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I purchased a new prop and the manual calls for 17 lb torque. Without a torque wrench I estimate that is finger tight and then tighten until the cotter pin lines up. Any suggestions?
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Old 08-01-2010, 18:35   #14
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I purchased a new prop and the manual calls for 17 lb torque. Without a torque wrench I estimate that is finger tight and then tighten until the cotter pin lines up. Any suggestions?
Sounds exactly right to me. Let us know the result with your new prop!
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Old 08-01-2010, 18:43   #15
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Quote:
Originally Posted by landonshaw View Post
I purchased a new prop and the manual calls for 17 lb torque. Without a torque wrench I estimate that is finger tight and then tighten until the cotter pin lines up. Any suggestions?
Or make your own torque wrench by hanging 17 pounds of weight on a line 12 inches out from the center of the nut.
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